My two latest spectra taken from "OHP station" (now we are at Juan les Pins
) - the nebular phase quick coming :
I have a important question for the observers who calibrate spectra in absolute value. Are you using a narrow slit or a slitless slit (large photometric slit) ?
As you know, the presence of atmospheric chromatism (the nova is not very high above the horizon from France) with a narrow slit can cause significant errors on the shape of the continuum.
In my case I use a refractor, which adds the problem of instrumental chromatism (although the FSQ106ED is apochromatic). The telescopes equipped with focal reducers can know also difficulties (less in general, but be careful ...)
For me these two phenomena and the setup produce high spectral distortion in the form of the continuum for the nova. For example, the ratio of a slit spectrum and a slitless spectrum (Alpy600 180 microns photometric slit) of the same star nearly the same time. The aspect of the continuum is very changing between the two situations !!!
To correct this dramatic (but ultimately classic) problem, I intensively exploited the photometric slit during the OHP run. For information, here I describe quickly the adopte protocol, I named "flash photometric correction" :
Step 1 : observation of a standard star (Miles) by using narrow slit (here 18 microns wide and a relatively short exposure time -> standard processing -> check and/or extraction of wavelength calibration function ("S0" spectrum)
Step 2 : observation of the same standard star by using the photometric slit (180 microns wide) - important point : no response correction applied (nearly the apparent spectrum view by the detector). Relatively short exposure observation (i.e. "flash"), typically 2 to 5 minutes exposure time. The result is the "S1" spectrum.
Step 3 : calculate the ratio S1 / "MILES" ("MILES" = the expected true profile is from the Miles data base). The result is the "PSR" profile (Photometric Spectral Response Profile).
Step 4 : observation of the target objet (here the nova) by using the photometric slit - important point : no response correction applied at this stage - "flash" procedure (short exposure time, the idea is to obtain the global acceptable form of the continuum and eventually absolutes flux infos). Result is the "S2" spectrum.
Step 5 : compute the ratio S2 / PSR -> "S3" spectrum. The S3 is the probable true spectral profile of the target (but some noisy because short integration time and degraded spectral resolution because use of a slitless mode).
Step 6 : observation of the target object (here the nova) by using a normal method : narrow slit and long exposure for the desired SNR. Do not applied here response correction. The result is the intermediate "S4" science profile of the target.
Step 7 : Compute a spectral correction term, the SSRT (for Slit Spectral Relative Transmission) : SSRT = S4 / S3.
Step 8 : smooth the SSRT profile (ISIS "continuum" tools for example -> remove hight frequency artifact) give the SSRT2 function.
Step 9 : Compute the "S5" final science profile : S5 = S4 / SSRT2.
The final equation is :
S5 = S4 x smooth[ (S2 x MILES) / (S1 x S4) ]
The procedure may seem long and tedious, but this is not really the case and it can be automated (you have to observe S1, S2 and S4 spectrum). This is the rather rigorous method that allowed me to obtain relatively correct spectra of nova during the "OHP campaign" by using a small refractor (but remember, a SCT telescope can be also affected).
The subject is important and deserves probably an exchange of opinions.
Christian B